Engineering notes
How to choose a sensor for a DDC: signal, range, supply
Choose a BMS sensor from the controller input: NTC, 0–10 V, 4–20 mA, dry contact. Match range and supply.
Three facts must match before you order
The wrong sensor type is a common reason a system “has software but cannot control”. Open the controller manual and read three lines: input type, measuring range, sensor supply.
1. Signal type
- Resistance (NTC, PT1000): common for room temperature. Use only a resistance input.
- 0–10 V: many air sensors and modulating valves. Needs 24 V and a voltage input.
- 4–20 mA: long runs, noisy rooms. Current input; sometimes a burden resistor.
- Dry contact: run/stop, fault, float — a digital input, not analog.
Do not buy “the same ceiling shape” before you know whether the output is mA or V.
2. Range
Chilled-water pipe temperature is not exhaust-air temperature. A 0–50 °C probe on a 90 °C pipe will read wrong or fail. 0–2000 ppm CO2 is enough for offices; a crowded meeting room may need a wider range if the consultant asks.
3. Supply and environment
24 V AC, 24 V DC or 230 V. Wall, duct or outdoor. A cold, wet plant room is not a corridor. A domestic wifi sensor does not replace an industrial probe on a technical panel.
Replacing a live device
Photograph the old label and the terminals on the controller. Note wire colours. If the exact model is gone, HTECH only proposes an equivalent with the same signal and range.